An ultramassive white dwarf in Eridanus may have escaped
from the Pleiades star cluster, say astronomers in England. If
so, the lost Pleiad was once a bright blue star that outshone all
the current cluster members. The discovery implies that the
white dwarf evolved from a single star, contradicting theories
that say ultramassive white dwarfs form only from the merger of
two lesser white dwarfs.

White dwarfs descend from stars born with less than eight
times the mass of the Sun. Stars in this mass range become
main-sequence stars, then expand into red giants. The red giants then
cast off their outer atmospheres, exposing their hot, dense
cores--which are actually white dwarfs. Over billions of years,
the white dwarfs cool and fade. The nearest white dwarf to Earth
is Sirius B, 8.6 light-years away.

Because red giants shed large amounts of material, a typical
white dwarf has only 60 percent of the Sun's mass. However, a
few white dwarfs contain more mass than the Sun. Astronomers
have proposed that many of these ultramassive white dwarfs result
when two less massive white dwarfs orbit each other and spiral
together. For example, if two 0.6-solar-mass white dwarfs merge,
they could create an ultramassive white dwarf with 1.2 solar
masses.

Paul Dobbie of the University of Leicester in England and
his colleagues studied an ultramassive white dwarf named GD 50,
which has 1.26 solar masses. GD 50 resides in northernmost
Eridanus, just across the border from Taurus, home of the
Pleiades star cluster. Furthermore, the astronomers say the
star's velocity through space matches that of the Pleiades,
suggesting the star was born there.

Moreover, GD 50 may be as young as the cluster--a further
sign it was once a member of the Pleiades. The cluster is about
125 million years old, which is 3 percent of the Sun's age. Most
white dwarfs are old, but GD 50 has been a white dwarf for only a
short time--about 60 million years.

Dobbie and his colleagues deduced the star's youth from its
high temperature. Since white dwarfs cool as they age, the
hottest white dwarfs are the youngest. GD 50's temperature is
41,550 Kelvin, which makes it as hot as a star of spectral type
O. If, prior to becoming a white dwarf, the star spent an
additional 65 million years as a main-sequence star and red
giant, then its age would equal that of the Pleiades. The
astronomers sketch out the star's evolution as follows:

Blue B-type Main-Sequence Star ===> Red Giant ===> White Dwarf.

"This result may represent the first direct observational
evidence that single star evolution can produce [ultramassive]
white dwarfs," write Dobbie and his colleagues in a paper to
appear in Monthly Notices of the Royal Astronomical Society.
That's because if GD 50 had arisen instead from the merger of two
lesser white dwarfs, it would be much older than the
Pleiades--since those two white dwarfs would have descended from less
massive stars, which take longer to die.

"I think it's an exceptionally interesting result," says
James Liebert, a white-dwarf expert at the University of Arizona
in Tucson who was unaffiliated with Dobbie's team. "It's not
completely airtight, but I'd say it's a pretty strong case."
Liebert puts the probability that GD 50 was born in the Pleiades
at 80 to 90 percent.

If GD 50 was once a Pleiad, why did it leave the cluster?
Dobbie and his colleagues say the star may have passed too close
to another Pleiad, whose gravity flung it out of the cluster and into the
void. Or perhaps, when GD 50 was a red giant, it lost more mass
in one direction than another, causing the star to recoil in the
opposite direction.

Although the white dwarf and the cluster have similar
motions through space, they reside at different distances from
Earth. GD 50 is about 100 light-years from us, whereas the
cluster is 435 light-years from Earth. Nevertheless, the
astronomers note that a mere 2 kilometer-per-second velocity
difference would create just this separation after only 55
million years.

The Pleiades star cluster is visible to the unaided eye on
early evenings in late autumn and winter. It is sometimes called
the "Seven Sisters," and legend has long told of a lost sister.
However, if the story refers to an actual star, that star is
probably not GD 50. GD 50 last shone brilliantly 60 million
years ago--before the human race arose.

As a white dwarf, GD 50 has such enormous surface gravity
that a pebble dropped from three feet above the surface would
smash into the star at 11,000 miles per hour. An American dime
as dense as GD 50 would weigh 2,600 pounds.

Ken Croswell is an astronomer and author. His book Magnificent Universe features a 10" x 14" color photograph of the Pleiades, and his book for beginning star gazers, See the Stars, tells when and where to see Taurus, the home of the Pleiades.